[lldb/Scripts] Add script to replay multiple reproducers

Script to replay reproducers in parallel using the command line driver.
This is used for active replay (stage 1 as described in the RFC on
lldb-dev [1]).

[1] http://lists.llvm.org/pipermail/lldb-dev/2020-April/016100.html

Differential revision: https://reviews.llvm.org/D77608